Transmissible spongiform encephalopathies are incurable, fatal neurodegenerative diseases characterized by the accumulation of abnormal prion protein (PrPsc), neuronal cell death and vacuolation of brain tissue. The PrPsc protein is extractable from diseased tissue and is distinguished from endogenous PrPc by partial protease resistance and detergent insolubility. The transmissible agent is the PrPsc protein and it serves as a template for the molecular conversion of endogenous host PrPc into the abnormal PrPsc structural isoform. Host expression of PrPc is necessary for disease transmission, as ablation of the PrPc gene prevents disease whereas the over expression of PrPc followed by PrPsc challenge accelerates disease. The molecular events that mediate neuronal PrPc to PrPsc conversion, not simply accumulated PrPsc, appears to be the initiating factor mitigating the neurodegenerative disease process.
The emergence of robust animal, cellular and biochemical models of prion disease has prompted the evaluation of a wide array of chemical agents targeted for use in therapeutic treatment (10-12). Yet, none have proven clinically effective with only a few chemical compounds identified that can delay the onset of prion disease in animal models (13). The only effective prevention of prion disease has been achieved by ablation or knockdown of PrPc in transgenic animals (14, 15). Alternate approaches such as protein based strategies have focused primarily on immunomodulation (16-18), or in one study, a PrPc-derived beta-sheet breaker peptide was shown to disrupt PrPsc structure and shown to delay the onset of clinical disease in mice (19).
The use of synthetic peptides designed with intrinsic functional protein domains have been engineered to facilitate drug delivery (20), cell attachment (21), and tissue regeneration (22, 23). These biomolecular scaffolds exploit the molecular properties of natural protein sequences to mediate targeted cellular events (22, 24, 25). They offer distinct advantages over traditional pharmacotherapies because they are composed of normal biological constituents, devoid of animal contaminants, biodegradable and do not provoke immune or inflammatory responses (24, 26, 27). Moreover, some of these substrates can mimic the dynamic structural changes observed in protein mis-folding diseases (28-30). The use of these peptides to inhibit endogenous protein mis-folding by protein stabilization, competition, increased clearance or degradation may prove useful in the elucidation of molecular conversion events or therapeutic intervention.
Herein is described a self-assembling synthetic peptide composed of a 16-mer RADA repeat that significantly extends hamster survival when pre-incubated with 263K Scrapie prior to intracerebral inoculation. The RADA-peptide (RADA) forms a hydrated scaffold of beta-sheet nanofibers that supports cell attachment and differentiation (29, 31). We show an initial delay in the accumulation of PrPsc in brain of animals inoculated with RADA followed by a significant increase in total PrPsc by day 75 and at the time of sacrifice. We demonstrate dose-dependent binding of PrP with RADA and show that this interaction can be competitively inhibited with Congo red. The combined inoculation of RADA with PrPsc results in a delay in clinical Scrapie symptoms and significantly extends animal survival. We postulate that a physiochemical interaction of PrPsc with RADA results in a molecular complex that alters PrPsc distribution and thereby impedes the efficacy of prion transmission and disease progression.